The present invention relates to a tool-holder for milling machines, to a milling machine, and to a milling procedure
The tool-holder, machine and procedure according to the present invention allow milling, bevel and radius cuts or other machining passes to be made automatically and to extremely close tolerances, even in situations where the exact geometry of the surface presented by the work is not known. More particularly, the invention will allow controlled bevel/radius passes to be performed on corner edges that have been generated previously by milling to known geometrical parameters, but are referable only to a surface of which the position cannot be determined by machine references.
The invention finds application preferably in the art field of CNC multi-axis machine tools as used in the aircraft industry, typically, to carry out milling, beveling and radiusing operations on the panels of wing structures and fuselage sections. This said, the invention also finds application to advantage in other sectors of industry, such as vehicle manufacturing.
In aircraft building, conventionally, openings have to be cut in fuselage panels to provide the surrounds for windows and for cargo doors. These openings are produced by a milling operation that involves piercing the panel and cutting out a hole of which the shape will match the outline of the window or door to be installed.
The subsequent installation of the window or door must be accomplished in such a way as to guarantee smooth continuity of the surface presented by the window and the surrounding panel, minimizing any bumps or ridges that could induce vortical airflows and undesirably increase the overall aerodynamic resistance of the aircraft. For example, the door seal must close flush with the panel of the outer skin, and accordingly, when the opening is cut, an edge or rim is generated where the outer surface of the panel meets the face of the cut, which will be beveled or radiused to accept the door seal. The width of the bevel must remain constant around the entire peripheral outline of the opening destined to accommodate the door or window, in such a way that the seal, which is of substantially constant thickness, will lie flush with the panels at any given point.
To produce the bevel, multi-axis milling machines of prior art design are programmed to follow the contour presented, for example, by the peripheral edge of the door opening on the curved surface of the panel in which the opening is cut. Knowledge of the panel surface is acquired by the multi-axis machine in the form of a mathematical equation that represents an ideal or nominal surface, and as such does not correspond exactly to the actual surface of the panel. In reality, whether a flat surface, as along segments of the wings, or curved as on the fuselage, the actual surface will always deviate from the nominal surface. Physically, the deviation is manifested as a series of undulations straying positively or negatively from the nominal surface and deriving from the process by which the panels are manufactured.
Consequently, whilst the motion of the toolhead on the multi-axis machine follows the programmed line of the nominal surface, the tool engages the actual surface, producing a bevel or a radius or a groove of depth that is different to the programmed depth and will not be constant, determined as it is by undulations of unknown value encountered along the machining path.
Already a factor affecting the preparation of aluminum alloy panels hitherto, the impact of this deviation is still more significant today in the case of panels made from composite materials such as carbon fiber. In effect, an error in machining depth on panels made of carbon fiber composite is particularly hazardous, as there is risk that the fibers can be broken, as well as the matrix, and the structural integrity of the panel jeopardized as a result.
Accordingly, the standard practice when forming bevels on carbon fiber composite is to use a hand tool with a depth stop, offered in contact to the irregular surface and rigidly associated with a blade that engages the corner edge being beveled. The operator thus slides the depth stop along the surface, while removing material from the corner edge with the blade to form the bevel or radius.
The applicant finds that the prior art in this field could be improved from various standpoints.
In particular, multi-axis CNC machines reflecting the prior art are not able to mill, bevel or radius automatically at constant depth while compensating for irregularities in a reference surface.
Performing such operations by hand is a laborious and slow process, and no longer compatible with the production tempos typical of present-day industry. Given the delicate nature of the work, moreover, especially when using fiber-reinforced composites, it must be entrusted to operatives in possession of particular skills, but prone nonetheless to human error. In addition, the carbon dust generated by cutting and machining composite materials of the type in question is most certainly harmful to any individual who may be performing these operations by hand.
The object of the present invention is to design a tool-holder for milling machines, also a milling machine and a milling process, such as will be substantially devoid of the drawbacks mentioned above.
In particular, the object of the invention is to provide a tool-holder, a machine and a process that will allow controlled milling passes to be carried out automatically on actual surfaces occupying any given position with respect to a nominal surface programmed into the machine.
Similarly, the object of the present invention is to provide a tool-holder, a machine and a process that will sink grooves, bevel or radius edges and remove burrs or flashes automatically, while also maintaining a constant machining depth relative to the actual surface of the work, especially in the case of pieces fashioned from composite material, preferably containing carbon.
A further object of the invention is to provide a tool-holder that can be utilized in conjunction with any numerically controlled machine, and in particular with multi-axis machine tools.